A major concern in the use of high strength metallic materials is the rapid growth of small flaws and defects as a result of the combined effects of tensile stresses and environment, something commonly referred to as stress-corrosion cracking. The resistance of material to such cracking can be quantified by fracture mechanics analysis. Fracture mechanics allows determination of the combined effects of stress and crack size in terms of a parameter K, the stress intensity factor. It has been shown that there exists a threshold value of K, called K.sub.iscc. This threshold value is determined for static loading conditions, and is considered to be an intrinsic material property which is utilized in structural design to compute allowable flaw size. Crack propagation rates as a function of applied stress intensity are used in combination with estimated or determined maximum initial flaw size to set inspection intervals for marine and other structures.
There are two generally accepted methods for determining K.sub.iscc, referred to as K-increasing and K-decreasing methods. With the increasing method, pre-cracked, cantilever-loaded beam specimens are maintained under constant applied load in the environment of interest for a fixed period of time that is specific to the material. Because K will increase rapidly with increasing crack length in the cantilever specimen, once crack growth begins it proceeds quickly to failure. Bracketing methods, involving specimens which fail and specimens where no stress-corrosion cracking is observed, are used to determine K.sub.iscc to about 10% precision. Little or no information concerning crack growth rates are obtained from cantilever bend tests.
A bolt-loaded compact tension [C(T)] test specimen is used for K-decreasing test methods. Such is generally described with reference to FIG. 1. The typical test involves fabricating a test specimen 10 from the material of interest into the shape shown in FIG. 1, and to certain specific dimensions. A typical test specimen constitutes a block of material having overall length, height and width dimensions of 3.2 inches, 2.48 inches and 1.0 inch, respectively. Alternate sized specimens can of course be utilized. A notch 12 is provided into specimen 10 from one end thereof. A threaded opening 14 extends from the top of specimen 10 downwardly to and transversely relative to notch 12. Notch 12 has an open spacing 15, and threaded opening 14 has a length 11 from the top exterior of the specimen to the notch. For purposes which will be apparent from the continuing discussion, this spacing and length in combination have a sum defining a first distance 19.
A hole 16 is provided through the thickness of specimen 10 in a precisely located manner to intersect through notch 12 as shown. A hardened steel tip 18 is separately provided and slidable within hole 16. It has an upper flat surface 20 which is coincident with the base of notch 12. Tip 18 is made of a material which is harder than the material of specimen 10. A threaded loading bolt 22 is received within threaded opening 14 for loading the specimen. Bolt 22 is threaded inwardly until its inner flat end bears against tip surface 20, upon which the specimen begins to experience load. The value of K can be computed from the measured value of the crack mouth opening before and after load, with such being recorded by an electronic crack mouth opening displacement gauge. Typically at this point, the specimen is inserted into the aggressive environment. This will typically lower the fracture energy of the material causing a crack 33 to grow under fixed displacement provided by loading bolt 22 against the tip.
Because the crack opening displacement is constant during this type of test, both load and K decrease as the crack extends. A specimen is loaded to a high value of K so that crack growth begins quickly and continues until K decreases to K.sub.isoc, when crack growth arrests. Once crack growth can no longer be observed, the test is effectively finished. Crack length and standard fracture-mechanics equations are then utilized to computer K.sub.iscc.
Unfortunately, the typical industry standard methods for determining final load include unloading and reloading the specimen at the end of the test. This incurs time, and hence money, and even then only one load data point at or near the end of the test is attained. It would be much more cost-effective to simply be able to record the arrest load or the entire load history during crack growth in situ during test.
Many different schemes have evolved for estimating bolt-induced specimen load in situ. One technique is to provide a strain gauge on the end surface of the specimen opposite the inner crack tip, such as at location 31. The strain during test is correlated with the estimated applied bolt-load through a secondary calculation performed in an approximating manner. This analytic method, commonly referred to as specimen compliance, breaks down when more than one crack initiates from the initial specimen crack, or substantial crack front curvature occurs.
Another method involves placing a strain gauge upon the load tup. Again, some form of calibration must be performed between the measured tup strain and applied load to the tup. Even this calibration is not accurate due to complex three-dimensional stress fields provided on the tup from Hertzian contact between the rotating bolt and non-rotating tup. Further, this Hertzian contact is impacted by any frictional unloading hysteresis effects that occur during the crack growth process as the energy in the specimen is transferred into growing the crack.
Yet another method uses an instrumented bolt having an internal hollow portion and strain gauge provided therein. Calibration must be conducted to account for the above-described Hertzian contact stresses.
It would be desirable to improve upon prior art methods and devices for determining crack propagation and load in bolt-loaded compact tension test specimens.